Ferromagnetic element with temperature regulation

ABSTRACT

A ferromagnetic element has current passed through it either by direct electrical connections or by induction. The current through the ferromagnetic element may be far greater than is necessary to heat the element above its effective Curie temperature. As the element is heated and is passing through its effective Curie transition (that is its temperature is rising from below its effective Curie temperature to, or above, its effective Curie temperature) the change in permeability of the element is sensed and the current through the element is cut off. The element then cools. When the temperature falls below the effective Curie temperature, the full current is restored. The heating and cooling process repeats itself indefinitely. The result is that the element is maintained at its effective Curie by a pulsating current fed to the element. The Curie transition may be sensed by directly sensing changes in permeability as by an auto-transformer winding, on the element; or by sensing changes in the power to the element by reason of the change in resistance of the element as it passes through the effective Curie temperature.

RELATED APPLICATION

.[.This application is.]. .Iadd.The original Letters Patent 4,769,519,issued from an application Ser. No. 3,288, filed Jan. 14, 1987 which wasa continuation in part of prior copending application Ser. No. 749,637,filed June 28, 1985, entitled Temperature Controlled FerromagneticElement now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to ferromagnetic elements possessing temperatureregulation when electrically heated.

It is old and well known to regulate the temperature of a ferromagneticelement by passing a radio frequency current through it. The currentheats the element to its effective Curie temperature where due to achange in permeability of the element, the power drawn by the elementdeclines, and therefore the device will hold its temperature constant.In some prior art devices, the skin depth within which the radiofrequency travels increases (and the resistance of the element to theradio frequency current) and the permeability of the element declines asthe temperature of the element approaches its effective Curietemperature. The "effective" Curie is the temperature at which thedevice regulates its temperature and is 50°-100° C. below the actualpublished Curie temperature. Hereafter, when reference is made to theCurie temperature it should be understood that the reference is to theeffective Curie, unless otherwise stated. The current may be fed throughthe ferromagnetic element directly, as by electrical conductorsconnected between the element and a source of current, or by induction.However, the known prior art employing a pure ferromagnetic element hasthe drawback that it will not hold the temperature constant over a widerange of cooling loads.

An improvement in the aforesaid temperature regulation method is shownand described in U.S. Pat. No. 4,256,945, issued Mar. 17, 1981, toPhilip S. Carter and John F. Krumme, entitled Alternating CurrentElectrically Resistive Heating Element Having Intrinsic TemperatureControl. This patent teaches that the temperature regulation may beimproved if the ferromagnetic element surrounds a copper substrate.Below the effective Curie the current is driven into the ferromagneticsurface layer by strong skin effect forces. When the temperature risesto the effective Curie temperature the skin effect is not strong due tothe change in permeability of the ferromagnetic material, and at leastsome of the current retreats into the copper. This results in a sharpdrop in power, since the current is held constant throughout theprocess. Hence, it is possible to design such a device that holds itstemperature constant over a wider range of thermal cooling loads thanwas the case with the ferromagnetic element.

The Carter-Krumme patent in col. 7 states the effectiveness of thedevice in terms of R_(max) where R_(max) is the resistance of the devicebelow Curie and R_(min) is the resistance of the device above Curie.

The Carter-Krumme patent teaches that the preferred frequency range is 8to 20 MHz.

SUMMARY OF THE INVENTION

With the present invention a pure ferromagnetic element is preferablyused; although a composite element such as that taught by theCarter-Krumme patent could be used.

According to the present invention, radio frequency current, preferablyin the general range of 5 to 20 MHz is passed through the ferromagneticelement, either directly or by induction. The amplitude of the currentis selected so as to heat the element well above its effective Curie. Anadvantage of this invention over the prior art is that it may employ amuch larger current than was feasible with the prior art, and,therefore, the load may be brought to its effective Curie more rapidlythan with the prior art. The current will quickly increase thetemperature of the element to its effective Curie. As the element isincreasing in temperature through its Curie transition the permeabilityof the ferromagnetic element will drop sharply. This sharp drop issensed, and, when sensed the current to the element is cut-off. Theelement then cools below the effective Curie and the current is restoredso as to again heat the element to its effective Curie. The process thenrepeats itself, hence a pulsating large current is fed to the element insuch a manner as to hold its temperature fairly constant. The sharp dropin permeability may, according to this invention be sensed in severaldifferent ways. On such way is to have the winding of anauto-transformer around the ferromagnetic element. Another way is tosense the change in power drawn by the ferromagnetic element, since thedevice may be so designed that the power will decline when thepermeability declines. When the power sharply declines the current iscut-off for a brief period and is then restored.

Arrangements according to the invention will now be further described byway of example, and with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a prior art arrangement wherein the RFcurrent is fed through a ferromagnetic element by direct electricalconnection thereto.

FIG. 2 is a schematic diagram of a prior art arrangement wherein the RFcurrent is fed through a ferromagnetic element by induction.

FIG. 3 is a graph of the temperature regulation of the devices of FIGS.1 and 2.

FIG. 4 is a schematic diagram of one form of the present invention.

FIG. 5 is a schematic diagram of another form of the present invention.

FIG. 5A is a modified form of FIG. 5.

FIG. 6 is a schematic diagram of still another form of the presentinvention.

FIG. 6A shows a modified form of FIG. 6.

FIG. 6B a schematic diagram of a modified form of FIG. 6.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a prior art ferromagnetic strip B which may be 0.010inches thick and 0.2 inches wide, composed of nickel-iron alloy having apermeability of over 100 and an effective Curie temperature in the rangeof 150° C. or more. The constant current power supply PS is capable ofdelivering sufficient power to the strip to heat it well above theeffective Curie temperature, for example 70° C. above the effectiveCurie. If then the current is turned on, the strip B will be heated totemperature T (FIG. 3) which is say 70° C. above the effective Curie C.If now a source of cooling fluid, for example gaseous carbon dioxide ispassed over strip B in progressively increasing quantity the temperaturewill fall along line E to level C and will remain there until ultimatelythe cooling is so great that the temperature will fall off along tailline D.

FIG. 2 illustrates a similar prior art arrangement in which current isinduced in ferromagnetic strip B by induction. This device will functionin the same way as the device of FIG. 1.

In FIGS. 1 and 2, if the amplitude of the constant current is reduced soas to reduce the initial temperature T, say from 70° C., above theeffective Curie temperature, to say 10° above the effective Curietemperature, the flat part C of the curve will be greatly shortened andtail D will occur at a much smaller cooling rate.

The present invention avoids the portion E of the curve above theeffective Curie temperature and also either avoids tail line D or atleast postpones it to such a high cooling rate that it is no problem.

Referring to FIG. 4, a ferromagnetic strip or bar 10 has a diameter orthickness of at least several thousandths of an inch. The configurationof element 10 may vary depending on the desired end use. For example, ifthe end use is a soldering iron, element 10 may have the shape of asoldering iron.

A small pick-up coil 11 is adjacent to, or around a part of,ferromagnetic strip, bar or rod 10. Coil 11 will function as anauto-transformer. The left half of the coil 11 is the primary and theright half of the coil 11 is the secondary. When the ferromagneticmember 10 is below Curie the primary of coil 11 is fed by a 60 Hzalternating current source 19 such as the secondary of a smalltransformer fed by a 60 Hz power line. The voltage of source 19 may bein the range of 8 to 24 volts. The control relay 16, 17 will beenergized by the voltage or current induced in the secondary ofauto-transformer 11 and will close the circuit to ferromagnetic member10 when the ferromagnetic member 10 is below Curie. That is, belowCurie, the ferromagnetic member 10 has high permeability and currentwill be induced in the secondary of auto-transformer 11. The secondaryof auto-transformer 11 applies an a.c. voltage across wires 12 and 14.This voltage is rectified by rectifier 15 and feeds relay coil 16,attracting armature 17 to close a circuit from source 18 throughferromagnetic member 10. The source may be in the range of 5 to 20 MHz,for example, and feeds sufficient RF current through member 10 to heatit well above Curie. As the member 10 is heated near or above Curie theauto-transformer 11 is no longer effective since the permeability ofmember 10 has dropped toward unity, hence the voltage in the secondaryof the autotransformer falls. Then, the relay coil 16 is deenergized andarmature 17 opens the circuit under the pull of spring 20. Next, thecurrent to the ferromagnetic member 10 from R.F. source 18 is cut off.The ferromagnetic member 10 then cools and when its temperature fallsbelow Curie the autotransformer 11 again becomes operative due to thehigh permeability of member 10. The secondary of auto-transformer 11 nowputs out full voltage, the relay 16, 17 closes and current from source18 is again passed through member 10 to heat it to Curie. The cycle thenrepeats over and over.

Solid state controls may replace parts 15, 16, 17.

The member 10 may be of high permeability such as Invar, Alloy 42, or aternary alloy composed of 45% nickel, 46% iron and 9% molybdium.

The parameters such as the amplitude of the current from source 18, timedelay etc. of relay 16, 17 may be selected so that the relay 16, 17opens and closes rapidly (several times a second). In the solid stateversion, the time delay of the parts will be selected to get the properfrequency for the opening and closing of the solid state switchcorresponding to relay 16, 17. If then there is a high rate ofextraction of heat from member 10 the relay 16, 17 will be closed longerthan it is open etc. But if one section 21 of the member 10 has muchmore heat extracted therefrom than is extracted from other equally widesections, the section 21 will receive more heat from the current as willbe explained. In such case, the section 21 will remain far below Curieand will not rise above Curie when the relay armature 17 is closed.Therefore, the skin depth of the current in section 21 will remainsmaller than for the remainder of member 10. Hence, section 21 will havehigher resistance per unit of length than the rest of member 10. Sincethe same current traverses the entire length of member 10, section 21will get more heat per unit length, and thus provide more heat to offsetthe fact that there is greater extraction of heat from section 21.

R.F. source 18 may be a constant current source but this is notnecessary. The fact that it is disconnected from the load above Curie issufficient control over the current.

A key point is that means are employed to detect the transition frombelow Curie to Curie and in response to detecting that transition thecurrent thru the ferromagnetic bar is cut off. If the device is arrangedto cycle on and off, and if the "off" periods are kept of short, thedevice should hold its temperature quite constant.

In connection with FIG. 4, it is preferable for the relay 16, 17 tocompletely disconnect the source 18 from the ferromagnetic strip 10.However, it would not depart from the broader aspects of the inventionto reduce the current to the ferromagentic element 10, when relay 16, 17opens, instead of cutting the current clear off. This may beaccomplished by placing a resistor across the contacts of relay 16, 17.

FIGS. 5 and 6, illustrate a different way of sensing the Curietransition. In these figures the change in power, that occurs when thetemperature increases through the Curie transition is sensed, and inresponse to sensing that change in power, the current to theferromagnetic element is either cut-off or reduced.

In both FIGS. 5 and 6 the load 69 is the high permeability ferromagneticelement and may have the composition, shape, and size described above,or as desired for any given end use.

FIG. 5 illustrates a constant voltage power supply for use with theinvention. This power supply has conventional oscillator 50,conventional buffer 51, conventional driver 52, and conventional class Camplifier 53, stages. While a wide variety of such equipment isavailable, one suitable form is shown in The ARRL 1985 Handbook (62ndEd., 1985), published by the American Radio Relay League, Chapter 30,pages 30-24 to 30-26. A copy of the applicable pages of this handbookwas filed with the parent application Ser. No. 749,637 of which is acontinuation in part. The driver 52 has an input to key the same on andoff and this corresponds to the key jack J1 on page 30-24 of saidhandbook. Preferably, the driver 52 is keyed by the contacts of a smallfast electromagnetic or solid state relay (not shown) in a conventionalfashion; the relay coil being energized by the pulses on wire 67 fromtimer 66. Alternatively the output 67 of timer 66 may bias the driver 52off.

The linear power amplifier 54, is optional, and may be any, of many,suitable linear amplifiers, for example it may be the 140 Watt SolidState Linear Amplifier, shown on pages 30-27 to 30-30 of said ARRL 1985Handbook. See also the Motorola RF data Data Manual (3rd. Ed., 1983),pages 4-194 to 4-199. The output of linear power amplifier 54 is fedthrough resistor 61, which feeds impedance matching transformer 68 inwhich turn feeds the load 69.

The voltage at the output of the linear power amplifier 54 is heldconstant by the components 55-60 as follows. Resistors 55 and 56 form avoltage divider across the output of power amplifier 54. The diode 57feeds resistor 58, capacitor 81, and amplifier 59, so that the output ofthe latter reflects the voltage at the output of power amplifier 54.That output feeds power regulator 60 which may be Texas Instruments,INc. Type LM 117, described on pages 99 to 103 of The Voltage RegulatorHandbook published by Texas Instruments, Inc. A copy of the applicablepages of this handbook was filed with the parent application Ser. No.749,637, of which this application is a continuation in part. Thisregulator 60 controls the main power input circuit 70 to the Class Camplifier 53 to thus raise or lower the output voltage thereof asnecessary to keep the output voltage of linear amplifier 54 fairlyconstant. This regulator 60 has a built-in conventional standardreference voltage which is compared to the voltage at the output ofamplifier 59, and the regulator 60 then functions to control the inputvoltage to Class C amplifier 53 so as to hold the voltage at the outputof linear amplifier 54 constant. The voltage control elements 55 to 60and 81 are unnecessary in those cases where the voltage of the radiofrequency source remains sufficiently constant that elements 55 to 60are not necessary.

If now the impedance of ferromagnetic load element 69 drops due to arise in temperature into, or through, the Curie transition, the currentthrough resistor 61 increases and the voltage at the input of diode 62increases thereby increasing the voltage at the negative (-) input ofoperational amplifier 64, the output of which feeds timer 66 with adecreasing voltage which in turn opens the keying circuit of driver 52turning off the driver 52, the Class C amplifier 53 and the linear poweramplifier for a time interval between 0.1 and 0.5 seconds; this timeperiod being manually adjustable by varying said timer 66. At the end ofthe selected time interval the driver 52 is no longer turned off by asignal on wire 67 and hence the driver is turned on, and power to theload 69 resumes. Current will again flow through resistor 61 to feed theload 69 and when the load impedance again drops the above process willrepeat itself shutting off the power. In this way the power to the load69 will pulsate as required.

Timer 66 has a built in standard reference voltage which is comparedwith the voltage at the output of amplifier 64, and the timer 66 istriggered to start its time period when the voltage at the output ofamplifier 64 becomes negative with respect to the standard referencevoltage of timer 66. When this happens the timer 66 applies a pulse towire 67 to cut-off all power at the output of linear amplifier 54.

The timer 66 of FIG. 5 may be Type 555 manufactured by TexasInstruments, Inc., and the manufacture's data sheet for this timer 66 isbeing filed with said parent application Ser. No. 749,637, of which thisapplication is a continuation in part. When this form of timer is usedthe input signal is fed into the Trigger (pin 2) of the timer 66.

The impedance matching transformer 68 in FIG. 5 may be designed and/orselected according to conventional practices such as those described insaid Motorola RF Device Data manual pages 4-145 to 4-153, or said ARRL1985 Handbook, FIG. 44, page 30-28.

The resistance values of the various resistors for FIG. 5 may be asfollows; it being understood of course that changes are necessary fordifferent designs:

    ______________________________________                                               Resistor                                                                             Ohms                                                            ______________________________________                                               55     1000                                                                   56      10                                                                    58     5600                                                                   61     0.01                                                                   63     5600                                                                   72     1000                                                                   73     5900                                                                   74     56000                                                                  76     7800                                                                   77     20000                                                           ______________________________________                                    

Capacitors 75, 80 and 81 may have a capacity of 0.001 mfd.

The 555 timer is actuated to start its time period when its triggerinput is fed with a declining voltage that falls below the built-insmall positive threshold of the timer 66. This condition is met in FIG.5 assuming that the amplifier 64 is biased to provide the desiredtrigger voltage. As soon as the declining output voltage of amplifier 68passes the threshold of timer 66 the radio frequency power is cut offand therefore the current through resistor 61 falls to zero. Therefore,the output of amplifier 64 rises above the threshold of timer 66 andremains there until not only the pulse at the output timer 66 expiresbut thereafter until there is a sufficient increase in the currentthrough resistor 61 to again trigger timer 66 to start a new timingperiod. The above overall cycle then repeats itself, providing apulsating or intermittent current to the load 69 as required to providea constant temperature.

If a simple system is desired the parts 64, 65, 72, 73 and 74 may beomitted and the output of rectifier 62 fed directly to driver 52 to biasit off (or open a relay in its keying circuit) when the voltage atrectifier 62 rises. Further simplicity may be achieved by omitting theconstant voltage regulating circuit 55 to 60 and 81 when the powersupply 5-54 is of the usual type which has a fairly constant voltage atits output.

FIG. 5A is a further modified form of FIG. 5 in which the radiofrequency source 83 has very low power compared to the source 54 of FIG.5, and the main heating power is supplied by the d.c. or 60 Hz source 90compared to the source 54 of FIG. 5. The frequency of source 83 is highenough in the megahertz range so that its output current increasessubstantially when the high permeability ferromagenetic strip 85increases in temperature and approaches the effective Curie temperature.If desired, the output voltage of source 83 may be controlled, or heldconstant, by a voltage regulating circuit 84 that is the same as orsimilar to the circuit 55 to 60 of FIG. 5; however in many cases theregulator 84 is not necessary since a simple low power r.f. signalgenerator usually has a fairly constant output voltage without aregulator. The current from source 83 is for control purposes and neednot, and usually is not, sufficient to substantially heat ferromagneticelement 69. The main source of heating current for element 69 is thesource 90 which may operate as any frequency but to save cost it wouldpreferably be a direct current source or a low frequency one such as 60Hz. Source 90 feeds the ferromagnetic element through the contacts 87 ofnormally closed relay 86, 87 which may be an electromagnetic relay (witha spring normally biasing it open) or its solid state equivalent.Capacitors 88 isolate the r.f. source 83 from the source 90, andinductors 89 isolate the source 90 from the radio frequency currents.Suitable components such as rectifier 85 and amplifier 85A are used tointerconnect resistor 61 to coil 86.

FIG. 5A operates as follows. The source 83 continuously passes a radiofrequency current through ferromagnetic element 69. When the element 69is well below Curie the element 89 has a relatively high resistance andthe voltage drop across resistor 61 is insufficient to open the normallyclosed relay 86, 87. Therefore, a large current from source 90 is fedthrough relay contacts 87 to the ferromagnetic element 69 heating thesame. When the element 69 is heated to the Curie transition thepermeability of element 69 falls, its skin depth increases and itsresistance decreases. Hence, the current through resistor 61 increases,and the current through coil 86 increases, opening the relay 86, 87. Thecurrent from source 90 is, therefore, cut off, the element 69 cools, thepermeability of element 69 rises, the resistance of element 69increases, and the current through resistor 61 falls. Relay 86, 87 thencloses. The above described cycle repeats itself over and over holdingthe current at element 69 relatively constant. Advantages of FIG. 5A arelower cost and lower possible radiation.

FIG. 6 will next be described.

The power generating stages 50 to 54 in FIG. 6 are essentially the sameas for FIG. 5, although they are controlled in a different way; andtherefore it is unnecessary to further describe those stages.

The current from the output of power amplifier 54 to the load 69 is heldconstant by components 60, 62, 63, 64, 70, 75, 76 and 77 as follows.When the current through resistor 61 increases the voltage drop acrossthat resistor 61 is fed to the input of operational amplifier 64 whoseoutput controls power regulator 60 (which may be Texas Instruments, Inc.Type LM 117 described above). The power regulator 60 controls thevoltage on wire 70 fed to Class C amplifier 53 to thus hold the outputcurrent of power amplifier 54 constant. As stated in connection withFIG. 5 the regulator 60 has a built-in standard reference voltage whichis compared with the voltage at the output of amplifier 64, and theregulator 60 functions to keep the two voltages the same and thus keepthe current at load 69 constant.

When the resistance of the load 69 (FIG. 6) falls, due to a rise intemperature into, or through, the Curie transition, the voltage at theoutput of power amplifier 54 also falls (due to the constant-currentcircuit 60, 62, 63, 64), and this voltage drop is sensed at the positive(+) input of operational amplifier 59. The change in output of thatamplifier is sensed by timer 66 which then places a short pulse on wire67 which opens the keying circuit of driver 52 and shuts off all powerat the output of power amplifierfor a predetermined time interval, forexample, between 0.1 and 0.5 seconds (the timer 66 may be provided witha manual adjustment to enable one to select the time interval he wants).When the time interval is up, the power is restored, and the abovedescribed heating-cooling cycle repeats itself. This repeating of thecycle continues as long as desired, with the result that a pulsatingcurrent is applied to the load 69 to hold its temperature constant. Thetimer 66 may have a built-in standard reference voltage which iscompared with the voltage at the output of amplifier 59. When thevoltage at the output of amplifier 59 becomes negative with respect tothe reference voltage, the timer 66 cuts-off driver 52 for apredetermined time interval as explained in connection with FIG. 5.

If the 555 timer, described above, is employed for timer 66 of FIG. 6 itis desirable for the voltage at the input (trigger) of the timer tobecome negative which respect to the reference (threshold) voltage ofthe timer; and to then again return to a voltage above that of thereference (threshold) value before the expiration of pulse at the outputof the timer 66. This return voltage may be provided by one skilled inthe art in many ways. One way is to insert suitable means in the outputof amplifier 59 (FIG. 6) to produce a trigger pulse of proper shape totrigger the timer 66. Another way is shown in FIG. 6A wherein a feedbackcircuit comprising amplifier 66A and capacitor 66B provides a signal,from the positive going output pulse of timer 66, to the trigger inputof that timer. The capacitor 66B holds the voltage at the trigger inputof timer 66 above its reference (threshold) value for a sufficient timerperiod to allow the driver 52, Class C amplifier 53 and power amplifier54 to apply full power to the load, so that the output voltage ofamplifier 59 will be high enough to hold timer 66 off as long as thetemperature of element 69 is below the Curie transition.

A system embodying the feedback circuit 66A of FIG. 6A works in the sameas the circuit of FIG. 6 except that the feedback circuit of FIG. 6A hasbeen added. Such a system operates as follows. When the load 69 is belowCurie the current to it through resistor 61 is held constant by theparts 60, 62, 63, 64, 70, 75, 76 and 77. When the temperature of theload 69 approaches Curie the resistance of the load declines and thevoltage fed to the positive (+) input to amplifier 59 declines. When theoutput voltage of amplifier drops enough so it becomes negative withrespect to the positive reference (threshold) voltage at the triggerinput of timer 66, the timer 66 produces a pulse on its output turningoff driver 52 and cutting off current to the load for the predeterminedtimer period for which the timer is set. At the same time, the voltagethrough amplifier 66A returns the voltage on wire 59A to a value abovethe reference (threshold) voltage of the trigger input of the timer 66and (due to capacitor 66B) holds the voltage at said trigger input abovethe reference voltage for a period a little longer than the time duringwhich the timer 66 holds driver 52 off.

Therefore, at the expiration of the duration of the pulse output oftimer 66, the radio frequency signal generator 52, 53, 54 resumes itsconstant current output through resistor 61 to load 69. The voltageacross voltage divider 55, 56 is again high since the load 69 cooledsomewhat while the timer 66 held driver 52 off. Thus, full power is fedto load 69. The temperature of the load 69 again rises, causing thevoltage at the input of amplifier 59 to fall and driver 52 is again cutoff. The above cycle repeats itself indefinitely thus producing a seriesof pulses through resistor 61 to load 69 and holding the temperature ofthe load fairly constant. The upper temperature of load 69 is the Curietemperature and the lower temperature is determined by the referencevoltage of the trigger of timer 66. Thus, when it is stated that theload temperature is held constant it is means that it is held withinlimits such as just described.

The modified form of FIG. 6B has a constant current radio frequencysystem 91, 93, 94, 95, 96, 97 to sense the resistance of the load 69 toradio frequency currents, and thus sense the Curie transition. When thetemperature of the load 69 falls below Curie, a d.c. or low frequency(60 Hz) source 90 is connected across the load 69. This latter source 90is disconnected from the load 69 when the temperature rises to Curie.

The low power radio frequency source 91 has a constant current output.If the resistance of resistor 61 is sufficiently high it may keep theoutput current from source sufficiently constant that no furtherregulation is needed. If further regulation is needed, a regulatingcircuit 92, when used, may conform to the circuit 60, 62, 63, 64, 70,75, 76, 77 of FIG. 6. The power output of source 91 is insufficient toprovide significant heat to load 69, since source 90 is relied on toprovide the heating current for the load 69.

As was the case with FIGS. 5, 5A, 6, and 6A, the load 69 is aferromagnetic strip or element of such high permeability, and of suchsize that the radio frequency current from source 91 will have a muchgreater skin depth at Curie than at temperatures well below Curie. Inthis regard source 91 preferably has a frequency above 5 MHz. Hence, asthe temperature of load 69 approaches Curie, its resistance to the highfrequency current from source 91 declines enough so that the decline maybe sensed by elements 93, 94, 95, 96 and 97.

In FIG. 6B, when the temperature is well below Curie the resistance ofload 69 to the radio frequency current from source 91 is high.Therefore, the constant current from source 91 flowing through the highresistance load 69 places a large voltage across resistor 94. The highvoltage across resistor 94 is rectified by rectifier 95 and fed toamplifier 96 the output of which closes normally open electromagneticrelay 97, 98 or its solid stated equivalent. Hence, the d.c. or 60 Hzsource 90 feeds heating current to load 69. The heating current may bequite large so as to heat the load to Curie in a very short time, forexample a few seconds.

When the load 69 reaches Curie, its resistance declines, the voltageacross resistor 94 declines and the voltage across relay coil 97 isinadequate to hold the relay 97, 98 closed. The source 90 is thendisconnected from the load 69 until the relay 97, 98 again closes. Theabove cycle repeats itself supplying a pulse of current from source 90to load 69 each time the relay 97, 98 closes. The series of pulses ofcurrent holds the temperature of load 69 fairly constant.

The operational amplifier 59 and 64 of both FIGS. 5 and 6 may be TypeuA741M or uA741C, manufactured by Texas Instruments, Inc., and a datasheet describing these amplifiers was filed with the parent applicationSer. No. 749,637, of which this application is a continuation in part.

In connection with FIGS. 4, 5 and 6 it is noted that the current fed tothe load 10 or 69, as the case may be, is not limited by the permissibletemperature T (FIG. 3). The current that may be applied to the load 10or 69 may be much higher than is permissible with FIGS. 1 and 2 or withany other known prior art. If a very large cooling load is applied toferromagnetic elements 10 or 69, the heavy current to those elementswill be "on" a much larger percentage of the time than will be the casefor a small cooling load. For example, if the cooling load is light, theheavy current will quickly reheat the ferromagnetic load element 10 or69, after the current is restored by the closing of relay 16, 17 or bythe expiration of the same interval of timer 66. But if the cooling,load is very heavy the time period for heating the ferromagnetic loadelement after the current is turned on will be longer than was the casefor the light load.

Thus, with the present invention, instead of the curve T, E, C, D ofFIG. 3, which is typical of the prior art, the curve would consist of asingle horizontal substantially straight line at the effective Curietemperature C.

Another advantage of the invention over the prior art referred to above,is that it will work over a very wide band of frequencies. For example,the device of FIG. 4, will work, even if power supply 18 has a d.c.output or has an output frequency as low as 60 Hz or even lower. In sucha case the invention would lose the value of providing greater heat to alimited section 21 (FIG. 4), that is cooled more than other sections.

The feature of providing increased heating to a limited section such as21, is applicable to all forms of the invention (FIGS. 4, 5 and 6), ifthe frequency is high enough to provide the necessary change in skindepth. However, in connection with FIGS. 5 and 6 the frequency and thesize of the ferromagnetic element should be so related that there is asubstantial change in skin depth, due to the change in permeability, asthe temperature goes through the Curie transition if a section such as21 is to get added heat when it is cooled. A strip several thousandthsof an inch thick will meet this requirement in the 8-20 MHz range. Forany frequency the ferromagnetic load may be several skin depths thick,for example, to meet this requirement.

An advantage of the present invention over the prior art is thatimpedance matching poses no problem at least in some forms of theinvention. In contrast, impedance matching is a serious problem in theprior art; for example in said Carter-Krumme patent the resistance ofthe load may be 40 times as high below Curie as it is at Curie. Hence,if the impedance is matched at temperatures below Curie it is notmatched at the Curie temperature. This can result in large losses in thepower supply including any power transmission line for feeding the load.However, in the present invention the impedance need only be matched attemperatures below Curie since the current is turned off when thetemperature reaches Curie.

The change in skin depth during the Curie transition will result in achange in resistance of the load 69, which will result in a change inpower, which is sensed and used as a control parameter.

The invention has end uses whenever it is desired to hold thetemperature of a strip, rod, bar, or other configuration constant. Onesuch use for example is in soldering as it is often undesirable tooverheat apparatus being soldered. Hence, the ferromagnetic element 10or 69 may be all or part of an element being soldered, or it may belocated in contact with an element being soldered.

The ferromagnetic elements 10 or 69 may also be used as heaters to heatchemicals to make sure that chemical reactions occur at predeterminedfairly constant temperatures.

I claim to have invented:
 1. In a temperature regulating device,aferromagnetic element having a permeability well above one below itseffective Curie temperature and a permeability of about one at itseffective Curie temperature, first and second power supplies, said firstpower supply having a relatively small power output as compared to thepower output of said second power supply, means for supplying the poweroutput of said first power supply to said element, said first powersupply including means for producting an alternating current outputhaving a frequency so high that the skin depth of said alternatingcurrent flowing through said element is substantially .[.greater.]..Iadd.smaller .Iaddend.at temperatures below said effective Curietemperature than it is at said effective Curie temperature, means forselectively supplying the output of said second power supply to saidelement to heat the same, said second power supply having a sufficientpower output to heat said element above its effective Curie, and meanscontrolled by the change in skin depth of the alternating currentflowing in said element for selectively controlling the powerfrom saidsecond power supply to said element and thereby regulating thetemperature of said element.
 2. In a temperature regulating device asdefined in claim 1 in which said first power supply has a radiofrequency output,said second power supply having an output for feedingsaid element with current which flows through said element to heat saidelement without substantial skin effect.
 3. In a temperature regulatingdevice:a ferromagntic element having a permeability well above unity attemperature well below the effective Curie temperature of said element,and control means for applying pulses of electric current to saidelement with each pulse heating said element to .[.tis.]. .Iadd.its.Iaddend.effective Curie temperature; said element cooling betweenpulses, said control means including means for sensing the permeabilityof said element during each pulse and for terminating such pulse whensuch permability declines to a given value and for sensing thepermeability of said element after the cessation of each pulse and forproducing the next pulse only after said element has cooled so that itspermeability is above said given value.
 4. In a temperature regulatingapparatus as defined in claim 3, said control means comprising means forholding the temperature of said element constant by controlling theduration of said pulses.
 5. In a temperature regulating apparatus asdefined in claim 4, wherein the time interval between the end of eachpulse and the beginning of the next one is at least a predetermined timeperiod.
 6. In a temperature regulating device as defined in claim 3,saidmeans for sensing the permeability of said element including a sourcewhich passes radio frequency current through said element and respondsto the skin depth of that current to determine the permeability of saidelement to terminate each pulse when the permeability declines to saidgiven value and to produce the next pulse only after said element hascooled so that its permeability is above said given value.
 7. In atemperature regulating device as defined in claim 6:said pulses ofelectric current being an alternating current of a frequency much lowerthan the frequency of said radio frequency current.
 8. In a temperatureregulating device as defined in claim 7:said pulses of electric currentcomprising pulses of current the frequency of which is 60 Hz.
 9. In atemperature regulating device as defined in claim 6, said pulses ofelectric current comprising pulses of direct current.
 10. In atemperature regulating device as defined in claim 6, said electriccurrent falling to zero between pulses.
 11. In a temperature regulatingdevice as defined in claim 6, said electric current having a substantialamplitude between pulses.
 12. Temperature regulating apparatus forholding the temperature of a heating element substantially constantwhile such element is subject to varying cooling loads, comprising:aheating element of the type that when in use is subject to a varyingcooling load, said heating element including: a ferromagnetic elementhaving a permeability substantially greater than one when itstemperature is below its effective Curie temperature and a permeabilityon the order of one when its temperature is above its effective Curietemperature, power delivery means feeding an electric current, throughsaid element, of sufficient amplitude which if maintained continuouslyand indefinitely, would heat said element far above its effective Curietemperature, so that said element has a Curie transition characterizedby a charge in permeability of said element, and control means includingmeans for supplying power to said element in the form of a series ofpulses of current with each pulse rising to said amplitude, said controlmeans comprising means for sensing said permeability during each pulseand at least reducing said electric current to said element to terminatethe pulse in response to such pemreability falling to a give valueduring an increase in temperature in said element, said control meansrestoring said electric current to said element to start the next pulseonly when said permeability is above said given value due to cooling ofsaid element; whereby a series of pulse is produced, each of whichpulses reaches said amplitude, and each of which pulses is of greaterduration for larger cooling loads than for smaller cooling loads; saidpulses holding said element at said substantially constant temperature.13. Temperature regulating apparatus as defined in claim 12 in whichsaid power delivery means is directly connected by an ohmic connectionto said element to thereby pass said current through said element,saidelement having a current carrying portion, said current being analternating current varying at a rate relative to the size of saidelement so that the current is concentrated along at least one surfaceof said element below the effective Curie temperature and spreads deeperinto the element as the effective Curie temperature is approached sothat if one part of the current carrying portion of said element iscooled more than another part of said element the said one part willreceive increased heat due to the decreased skin depth of the current insuch one part.
 14. Temperature regulating apparatus as defined in claim12 in which said power delivery means comprises means for inducing saidcurrent in said element,said .[.elemenet.]. .Iadd.element.Iaddend.having a current carrying portion, said current being analternating current varying at a rate relative to the size of saidelement so that the current is concentrated along at least one surfaceof said element below the effective Curie temperature and spreads deeperinto the element as the effective Curie temperature is approached sothat if one part of the current carrying portion of said element iscooled more than another part of said element the said one part willreceive increased heat due to the decreased skin depth of the current insuch one part.
 15. Temperature regulating apparatus as defined in claim12 in which said sensing means comprises inducting magnetization B tosaid element and detecting the magnetic field H produced thereby. 16.Temperature regulating apparatus as defined in claim 12 in which saidsensing means comprises means for detecting a change in the power ofsaid current as the temperature of said element increases and saidelement undergoes at least part of a Curie transition.
 17. Temperatureregulating apparatus as defined in claim 16 in which said sensing meanscomprises means that responds to a change in voltage to detect saidchange in power.
 18. Temperature, regulating apparatus as defined inclaim 16 in which said sensing means comprises means that responds to achange in current to detect said change in power.
 19. Temperatureregulating apparatus as defined in claim 12 in which said sensing meansincludes means for restoring said current to said element after at leasta predetermined time interval following the time that said current wasat least reduced by said sensing means.
 20. The method of temperatureregulation, of a heating element subject to a variable cooling load,comprising:providing said element with ferromagnetic material having apermeability considerably greater than one below its effective Curietemperature and a permeability on the order of one above its effectiveCurie temperature, subjecting said element to at least two cooling loadsone of which is larger than another one, passing an electric current, ofsufficient amplitude which if fed through said element for a prolongedperiod will heat said element well above its effective Curietemperature, through said element to heat said element at least to atemperature high enough so that at least a part of the Curie transitiontakes place, sensing at least part of said Curie transition and reducingthe flow of said current through said element when at least part of saidCurie transition sensed, and restoring said current through said elementto said amplitude, said sensing and restoring steps recurringalternately to thus provide a pulsating current that holds said elementat about its effective Curie temperature and at all times prevents arise in the temperature of said element substantially above itseffective Curie temperature, said pulsating current comprising pulses ofgreater time duration for the larger cooling load than for the saidanother one.
 21. The method of temperature regulation of claim 20,comprising:said current being an alternating current, the frequency ofsaid alternating current being so related to the size and permeabilityof said element that the skin depth of the alternating current flowincreases when the temperature increases into the Curie transition tothus lower the effective resistance of said element to said alternatingcurrent, whereby the power fed to said element changes during the Curiestransition, said sensing step comprising sensing changes in the power ofthe alternating current fed to said element, to thereby sense at leastpart of the Curie transition during an increase in the temperature ofsaid element and thereupon reduce the power fed to said element therebylimiting further heating of said element.
 22. The method of claim 21 inwhich said power fed to said element includes the parameters of voltageand current one of which has less percentage variation than the other attemperatures of said element below and during the Curie transition,saidsensing step including the sensing of changes in the other of saidparameters to thus ascertain at least a part of the Curie transition andthereupon reduce the power fed to said element and limit the rise intemperature of said element.
 23. The method of claim 22 in which thecurrent has the smaller percentage variation, and the change in voltageis sensed to ascertain at least part of the Curie transition.
 24. Themethod of claim 22 in which the voltage has the smaller percentagevariation, and the change in current is sensed to ascertain at leastpart of the Curie transition.
 25. The method of claim 20 in which saidsensing step comprises applying magnetic induction to said element andsensing the resulting magnetic field in the element to thus determinewhen the element passes through at least part of the Curie transition.26. The method of claim 20 in which said sensing step comprisesdetecting the change in power fed to said element due to at least partof the Curie transition,providing a power source for supplying saidcurrent, and feeding said current to said element by an ohmicconnection.
 27. The method of claim 20 in which said current is restoredin at least a predetermined time period after it is stopped.
 28. Themethod of claim 20 in which an increase in the permeability of saidelement is sensed and said current is restored when such increase inpermeability is sensed.
 29. Temperature regulating apparatus, forholding the temperature of a heating element substantially constantwhile such element is subject to varying cooling loads, comprising:aheating element of the type that when in use is subject to a varyingcooling load, said heating element including: a ferromagnetic elementhaving a permeability substantially greater than one when itstemperature is below its effective Curie temperature and a permeabilityon the order of one when its temperature is above its effective Curietemperature, a radio frequency power supply feeding current, throughsaid element, of sufficient amplitude, that if maintained continuouslyand indefinitely, would heat said element far above its effective Curietemperature, so that said element has a Curie transition, said powersupply having an output and also having the parameters of both currentand voltage at said output, feedback means for holding one of saidparameters, when the current is on, substantially constant at saidoutput, and control means for sensing changes in the other of saidparameters for turning said radio frequency power supply off and on toproduce a series of pulses, said control means turning said power supplyoff when the permeability of said element falls to a given value due tothe heating of said element.
 30. Temperature regulating apparatus asdefined in claim 29, in which said control means includes a timer forholding said power supply off for at least a predetermined time after ithas been turned off by said control means.
 31. Temperature regulatingapparatus as defined in claim 30 in which said current at said output isheld constant.
 32. Temperature regulating apparatus as defined inclaimed 29 in which said voltage is held constant.
 33. The method oftemperature regulation, comprising,providing a ferromagnetic elementhaving a permeability well above unity at temperatures well below theeffective Curie temperature of said element, applying pulses of electriccurrent to said element with each pulse heating said element to itseffective Curie temperature, allowing said element to cool betweenpulses, sensing the permeability of said element during each pulse andterminating such pulse when such permeability declines to a .Iadd.given.Iaddend.value, and sensing the permeability of said element after thecessation of each pulse and producing the next pulse only after saidelement has cooled so that its permeability is above said given value.34. The method of temperature regulation as defined in claim 33 in whicheach of said sensing steps comprise passing a radio frequency currentthrough said element and responding to the skin depth at which saidcurrent flows in order to determine the permeability of said element.35. The method of temperature regulation as defined in claim 33 in whichthe current through said element is turned off between pulses.
 36. Themethod of temperature regulation as defined in claim 33 in which thereis a substantial current through said element between pulses.